The present invention generally relates to an apparatus and method for dispensing fluids and more specifically, to a gun driver diagnostic circuit for a fluid dispensing apparatus.
Pneumatic and electric fluid dispensers have been developed for dispensing applications requiring a precise placement of a fluid. Pneumatic dispensers have a significant advantage in that the pneumatic solenoid operating the dispensing valve provides a sufficient force so that the dispensing valve operation is essentially independent of the viscosity of the fluid being dispensed. However, pneumatic solenoids have disadvantages in that they often have a shorter life than electric solenoids, and the operation of the pneumatic solenoid is subject to less precise control than the electric solenoid in an electric fluid dispenser. Therefore, in some applications, electrically operated fluid dispensers are preferred over pneumatic fluid dispensers.
Generally, electrically operated fluid dispensers include an electromagnetic coil surrounding an armature that is energized to produce an electromagnetic field with respect to a magnetic pole. The electromagnetic field is selectively controlled to open and close a dispensing valve by moving a valve stem connected to the armature. More specifically, the forces of magnetic attraction between the armature and the magnetic pole move the armature and valve stem toward the pole, thereby opening the dispensing valve. At the end of a dispensing cycle, the electromagnet is de-energized, and a return spring returns the armature and valve stem to their original positions, thereby closing the dispensing valve.
With both pneumatic and electric dispensing guns, a driver circuit provides a drive signal to an inductive load, either a solenoid coil in a pneumatic gun or a gun-operating coil in an electric gun. Changing electrical characteristics of the load or output circuit connected to the driver circuit can result in inconsistent and improper operation of the coil being driven by the driver circuit and hence, the operation of the fluid dispensing gun is adversely affected.
For example, the output circuit may be improperly wired or, wires may be damaged and broken such that an open circuit or high resistance load is connected on the driver circuit. With such an open circuit condition, the dispensing gun fails to operate. Such a failure may occur any time that the dispensing gun is being commanded to operate and may go unnoticed by the user until defective or scrap product is observed. Thus, it is useful to be able to detect such a high impedance or open circuit condition and signal the user, so that corrective action can be taken.
In other situations, the driver circuit may be connected to a dispensing gun with a coil having an impedance that is mismatched to the output impedance of the driver circuit. While some impedance mismatching is acceptable, excessive impedance mismatching can result in an inconsistent operation of the fluid dispensing gun.
Consequently, there is a need for a diagnostic circuit for use with a driver circuit of a fluid dispensing gun that detects and alerts a user to higher output circuit impedances that can adversely effect the operation of the fluid dispensing gun.
The present invention provides a simple and reliable diagnostic circuit for a driver output circuit of a fluid dispensing gun that is sensitive to a wide range of high impedance conditions. The diagnostic circuit of the present invention is capable of signaling the user of high impedance characteristics in the driver output circuit that may result in an inconsistent operation of the fluid dispensing gun. The diagnostic circuit of the present invention also signals the user in the event the user attempts to use a dispensing gun presenting a severe impedance mismatch with the output of the driver circuit. The diagnostic circuit of the present invention is especially useful in providing signals, in a timely manner, that apprise the user of conditions that may result in an improper or inconsistent operation of the dispensing gun. Thus, the user can address the condition in a timely manner and reduce the production of defective or scrap product.
According to the principles of the present invention and in accordance with the preferred embodiments, the invention provides an apparatus for detecting a high impedance load on an output of a driver circuit. The driver circuit has a power switching circuit providing output signals to a coil, wherein each output signal results from a drive signal that has a waveform comprising a higher magnitude initial peak period followed by a lower magnitude hold period. The apparatus includes a diagnostic circuit connected to the driver circuit. The diagnostic circuit provides a high impedance error signal in response to detecting, during only the higher magnitude initial peak period of the drive signal, a voltage of the output signal exceeding a predetermined value.
In another embodiment of the invention, the driver circuit has a feedback circuit providing a difference signal as a function of a difference between an output signal and a corresponding drive signal. The diagnostic circuit provides a high impedance error signal in response to detecting the difference signal crossing a threshold magnitude. In one aspect of this invention, the diagnostic circuit provides the high impedance error signal in response to detecting the difference signal crossing a threshold magnitude during the higher magnitude initial peak period of a drive signal.
In a further embodiment of the invention, a method is provided for detecting a high impedance load on an output of a gun driver circuit electrically connected to a coil of a fluid dispensing gun. The method first applies output signals to the coil in response to corresponding drive signals, each drive signal having a waveform comprising a higher magnitude initial peak period followed by a lower magnitude hold period. Next, a high impedance error signal is provided in response to detecting a magnitude of an output signal crossing a threshold magnitude.
In a still further embodiment of the invention, a difference signal is produced as a function of a difference between each of the output signals and a corresponding drive signal. The high impedance error signal is then provided in response to a sampled difference signal exceeding a threshold magnitude. In one aspect of these embodiments, the high impedance error signal is provided in response to detecting the difference signal crossing a threshold magnitude during the higher magnitude initial peak period of a drive signal.
These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein.